Process for cooling a gas



Jan. 27, 1959 w. P. HENDAL 2,870,611

PROCESS FOR COOLING A GAS Filed Nov. 12, 1954 COOLANT V 10 13 2 GAS INLET HOT GAS GAS INLET HOT GAS COLD J GAS l5 LCOOLANT INVENTOR WILLEM PIETER HENDAL BY mawflpw HIS AGENT United States PROCESS FOR (IOOLING A GAS Application November 12, 1954, Serial No. 468,487

Claims priority, application Nether-tends November 20, 1953 1 Claim. (Cl. 62-5) This invention relates to a process for cooling a gas by expanding it in a vortex tube, and to an improved vortex tube for the purpose.

The so-called vortex tube, also called a Ranque tube or a Hilsch tube, appears to have been discovered first by Georges I. Ranque, the construction and some of the operating features of such a tube being described in U. S. Patent No. 1,952,281, issued March 27, 1934 to Ranque. A bibliography of publications on the vortex tube has been published by W. Curley and R. MacGee, In, in Refrigerating Engineering, vol. 59, 1951, pages 166 and 191-193. As set forth in the aforementioned patent and the prior art referred to in the mentioned bibliography, the vortex tube may include a chamber having the shape of a surface of revolution, such as a cylinder, this chamber having an inlet pipe for tangentially introducing the gas to be treated into a central region of the chamber. A gyratory motion is imparted to the gas as it enters the chamber, and means are provided to divide the gas into two concentric sheets moving along each other. If part of the gas is discharged at a point in or near the center line of the tube and another part at a point on the periphery of the tube, the result is that the former part has become colder and the latter part warmer than the entering gas. This phenomenon which occurs in the vortex tube during expansion of the gas will hereinafter be referred to as the heat separation effect. This heat separation effect results in the vortex tube (in an uncooled tube) becoming hot at least for a portion thereof and this will be referred to as the hot side of the vortex tube.

It has been proposed, according to German Patent 858,260 to increase the heat separation effect of the vortex tube by providing an apparatus in the tube by which the peripheral hot gas is separated from the central cold gas almost immediately in the vicinity of the inlet end; mixing of the hot with the cold gas would thus be avoided, and it would be possible to reduce the length of the vortex tube considerably. It is also proposed in the patent to cool the hot gas thus separated, and to mix led into the central part of the vortex tube itself and there,

mixed with the cold gas.

Now, in accordance with the present invention, it has been found that the heat separation effect of the vortex tube can be improved considerably. The means for accomplishing this improvement will be described with reference to the accompanying drawing, wherein:

Fig. I is a diagrammatic representation of a vortex tube provided for external cooling;

' hot gas outside the actual vortex tube.

inc

Fig. la is a view partly in cross-section of Fig. I taken at Ia-la;

Fig. II is a diagrammatic representation of a vortex tube provided for internal cooling; and

Fig. Ila is a view partly in cross-section of Fig. II taken at IIaIIa.

It has now been discovered that the heat separation effect is enhanced considerably by expanding a gas in a vortex tubewhich is empty at the place where the heat separation effect occurs and intensively cooling the tube. Accordingly, there are no structural means in the hot end of the tube for separating peripheral hot and central cold gases. Also, the cooling is so intensive that when approximately 15% to 20% of the gas introduced is transported from the vortex tube as hot gas, the absolute temperature of the hot gas is not more than approximately 15% and preferably not more than 3% higher than the absolute temperature of the gas introduced.

In this connection it is to be noted that although the cooling ribs of the apparatus described in the abovementioned German patent extend over part of the actual vortex tube, it is not intended to cool the vortex tube itself, but only the lengthening-piece, which amounts to an attachment only and effects no vortex tube heat separation effect itself. There is actually no appreciable coolin of the vortex tube per se.

The improvement in the heat separation effect by the.

practice of the present invention may be considered to be unexpected; those skilied in the art might have expected that intensive cooling of the hot gas flowing along the vortex tube would cause a disturbance in the process and/or in the vortex pattern occurring in the tube, and would be confirmed in this expectation by the contents of the German patent which prescribes cooling of the The cooling which is effected according to the German patent is caused by free convectionof air. Presumably this method of cooling was considered heretofore to be as effective as any other since it would serve no useful purpose to increase considerably the transport of heat from the wall of the tube to the outside as compared to the transfer of heat from the hot gases to the wall.

It has been discovered, however, that there is a very large transfer of heat from the gases to the wall of the tube, provided the flow in the tube is not disturbed; and then a useful purpose is served by improving the transport of heat from the wall of the tube to the outside. In the vortex tube proposed in the German patent, however, the flow is disturbed in the immediate vicinity of the inlet end of the gases by the presence of means for separating the hot gas from the cold gas and consequently no great transport of heat is possible in that case.

In connection with the fact that the above-mentioned separating means in the vortex tube of the German patent extends to a point at which the gases still have a considerable amount of kinetic energy (e. g. the point at which the heat separation effect occurs), the yield of cold, 'as measured either by the minimum temperature of the exiting cold gas stream or by the product of the fraction which exits as cold gas times the difference in temperature between it and the-entering gas, from the vortex tube according to the German patent is less than the yield from the tube in accordance with the present invention.

For carrying out the present process it is advantageous to use a vortex tube, as represented in Fig. I, which is provided with a cooling jacket 12 at least over the length of the empty part. Preferably an empty tube 11 is used (with inlet and discharge ends for the gases fitted. in a manneralready known) which is provided with a cooling jacket'over the whole of its length. The inlet 13- of the cold coolant, for example cooling water, is gengas-inlet or the discharge end for the cold gases The valve 16 provided inthe hot end of the tube isadapted ito control the proportion of gas which is withdrawn 'from the tube at the hot end, permitting the exit of a portion of the total gas; as hot gas, or none at all;

Referring to Fig.;II, there is shown a vortex tube 21 which is adapted to be cooled directly onthe inside by means-of acold. liquid flowing along the inner wall of the vortex tube, delivered theretoby coolant inlet 22 which is suitably placed near-thegasinlet 23'. As a result of the centrifugal force and the helical movement 10f thev gas the cooling'liquid moves along the inner-wall of. the-vortex tube in'the formof-a film. The cooling liquid-is discharged from thehot gas discharge end of ,the, vortex tube together with hot'gas discharged -therefrom. The cooling liquid is suitablydischarged from one or. more points, if desired, and when none of the gas is dischargedfrom the hot end, the cooling liquid is 'discharged. alone, except for that portion which is vaporized and. atomized with the gas stream and carried out with the cold gas stream.

It will be understood that both methods of cooling can be utilized at. the same time, with the same vortex tube being adapted for both external and internal cooling.

The. inlet end of the vortex tube, according to the invention; is preferably provided close to the discharge end for the cold gas, i. e. near the diaphragm Which defines the axial stream outlet.

The length of the empty part of the tube (the distance from the gas inlet to the central discharge for the cold gases is here left out of account) is u'suallylarge wit regard to the diameter of the tube, as follows:

If no gas is discharged on the hot side of the vortex tube the length. of the empty part is at least 6 times the diameter of the tube.

Very good results are obtained with alength 9 to IO'times the diameter, but the maximum effect is found tobe produced with' a still greater length approximately 18 to 20 times the diameter.

If, a portion of the gas is discharged on the hot side of the vortex tube the length of the empty part thereof may be somewhat smaller, e.. g. at least 3 to 4 times thediameter, the preferred length being at least 9 times the.diameter-of=. the tube. In this case, as in the case where nogas is discharged on the hot side, the maximum effect. to beobtained is produced when the tube is longer.

Inboth cases, therefore, the length may be 18 or more timesrthe diameter of the tube.

H'lihe .maximum quantity of cold is produced when between.l% and 30%, preferably approximately 20% 10f the gas. introduced is discharged on the hot side of the vortex tube. By. quantity of cold isr'neant the product of the..proportion (;r.) of thegas which is discharged as coldgas times thed-ifference: (A!) between its temperature and the temperature of the entering gas. In the present processyit issalso possible toremove no gas at all on the hot side, eg. when in a. closed system it is desired not tolose any gas, so that the whole amount of gas is produced as cold gas (the vortex tube then need have only the-one central or axial discharge opening).

Irr the process according: to the invention, it is possible toregulate and/or adjust the temperature of the effluent cold gas in a simple way by regulating the velocity of the coolant" often cooling water) flowing through the coolingjacket or along the inner wall of the vortex tube. The gasto-be introduced may originally be compressed and -expansion= may occur against the pressure of theatmosphere'; Since the heat separation effect depends on the: ratio:of initi'alto final pressure-which ratio often has an: approximate; value: of about -expansion' of the gasund'erxnormal or increased pressure against a vacuum gives a still better heat, separation effect. Expansion into a vacuum is suitable when, for example, two or more vortex tubes are connected in series, in which case the gas expands in stages.

Having described the invention and the various features of it which come into consideration in its practice, the application of the invention and the advantages to be derived from its practice will be set forth in the following illustrative examples.

Example I 30 kilograms of air per hour (temperature approximately 22 C.) were led at a pressure of 4.5 atmospheres into an empty vortex tube having an inside diameter of 10 mm, a length of mm.,,a .tangential rectangular inlet of 6.72 sq. mm., and a central discharge opening for cold gas of 6 mm. diameter. The vortex tube, which was provided with a cooling jacket, was intensively.

cooled with Water of a temperature of approximately 10 C. The entire amount of the air was obtained as cold gas having a temperature of l4 C. (no hot' gas discharge). 7

In contrast to the above, a similar operation with the exception that no cooling of the vortex tube except by: the adjacent air, resulted' in a maximum Wall tempera: ture of 200 C., and the temperature of the gas drawn off at the central discharge opening is. essentially equal. to the inlet temperature of the gas, i. e. about 22 C.

Example I! A mixture of carbon, monoxide and water vapor was converted to form carbon dioxide and hydrogen in-a plant at- 550 C. in twoconverters, each containingJO:

tons of a catalystof iron and chromium oxides.

T he'se converters have to be put out of operation fromv time to time to permit inspection, and for this purpose it is necessary to cool them to approximately 25 C. Cool.-

ing isfirst effected by steam and then with cold nitrogen: Formerly nitrogen of approximately 3 C. was. used. for this purpose, but now nitrogen of this temperature under a pressureof 6 atm. is ledintoan empty vortex. C., in a quantity of 1200 tube, cooled by Water of 16 cubic meters per hour.

The dimensio'ns of this vortextube are:

Inner diameter cm 7.6 Length; cin 190 Area of gas inlet sq'. cm" 3 Diameterof dischargefor col'd gas'es cm 4.8

Fifteen (15) percent of the gasin-troduced wasremoved from the hot side of the. vortex. tube by means.- ofa valve; in this way a maximumquantity of cold was produced. Th'eicooling of the tubes was of su'fiicient-i-ntensit-ivity to give a cooled nitrogen. stream of 30 C., while the hot gas had atemperatnre of approximatelyAG" C.

In order to prevent the occurrence of undesirable stresses in the material of the converter-s. at the beginning.

Example III The effect of theintensivity' of cooling. on theoperationof a vortex tube according. to the invention is seen from thefollowing tests. Compressed air. (6 atm. absi) was allowed toexpand into an empty vortexv tube, provided with a cooling jackenthe' dimensions'bein'g: as follows:

Inner diameter of the tube mm 10 Length of the tube ...mm..

195 Area of gas inlet sq. mm" 5.12 Diameter of central discharge for cold gases mm 6 Diameter of cooling jacket mm 16 Length of cooling jacket mm 200 The cooling water and the air introduced were at the same temperature (13.5 C.). The difference in temperature between the cold gases and the gas introduced is indicated by At; the temperature of the hot gases by z,,,.

(11) Approximately 15% of the gas introduced was drawn ofi on the hot side. When the quantity of cooling water was increased from 5 liters per hour to 200 liters per hour, At increased from 29.5 to 34 degrees centigrade, while t decreased from 45 C. to 225 C.

(b) No gas at all was drawn off on the hot side. When the quantity of cooling water was increased from 5 liters per hour to 500 liters per hour, At increased from 23.9 to 28.4 degrees centigrade.

From the above data it follows that with the present vortex tube the coefficient of heat transmission was of the order of 1000 kcal. per hour per square meter per degree centigrade. On the other hand, with the free convection cooling described in the German patent already referred to, this coeflicient is only of the order of kcal. per hour per square meter per degree centigrade, being only about one percent (1%) of the value in accordance with the present invention. Thus, the present cooling is therefore intensive as compared to the cooling by free convection. stantial advantage in the use of the present invention.

Example IV In order to demonstrate the material and undesirable effect of the presence of structural bodies in the vortex tube over the part where the heat separation eifect occurs, a concentric, open hollow tube with a smaller di-' Even a ten-fold increase would represent a sub-,

6 atm. abs. The temperature of the air was the same as that of the cooling water, namely, 14 C. The velocity of the cooling water was about 300 liters per hour.

(a) Approximately 15% of the hot gas was'drawn oflf. When the end of the hollow tube extended so far into the vortex tube that the empty part of the vortex tube had a length (l) equal to the diameter (d) of the vortex tube, t=6.5 degrees centigrade. When l=3d, t=26 degrees centigrade; when l=10d, t=33.5 degrees centigrade.

(b) No hot gas was drawn off.

When l=d, t=12 degrees centigrade; when l=6a', t=25.5 degrees centigrade; when l: 10a, t=28 degrees centigrade.

I claim as my invention:

In a process for producing cold in a gas stream utilizing a vortex tube wherein the gas stream is divided into a peripheral outer hot portion and a central cold portion and wherein there is a heat separation zone for tram..- ferring heat from the central cold portion to the outer hot portion, said vortex tube being unobstructed along its length where the heat separation effect occurs, the improvement comprising intensively cooling the gases of the heat separation zone by direct heat exchange with a liquid coolant flowing along at least a portion of the inner wall of the vortex tube enclosing said heat separation zone, and thereby facilitating the transfer of heat from the central cold portion to the outer hot portion, resulting in a lower temperature in said cold portion than obtainable in the absence of said flowing liquid coolant.

References-Cited in the file of this patent UNITED STATES PATENTS 2,519,028 Dodge Aug. 15, 1950 2,644,315 Iamieson July 7, 1953 2,650,582 Green Sept. 1, 1953 2,698,525 Lindenblad Jan. 4, 1955 2,720,091 Schelp Oct. 11, 1955 2,741,899 Von Linde Apr. 17, 1956 2,786,341 Green Mar. 26, 1957 FOREIGN PATENTS 708,046 Great Britain Apr. 28, 1954 759,440 Great Britain Oct. 17, 1956 858,260 Germany Dec. 4, 1952 1,045,871 France July 1, 1953 

